Constant lumen output control system

ABSTRACT

A constant lumen output control system for providing a constant lumen output throughout the life of a lamp at the mean or preset lumen level. The lumen con system ( 315 ) coupled to a lamp driver ( 310 ) initially reduces the power to the lamp ( 330 ) to prevent the lamp from being operated at power levels that result excess mean or preset lumen levels. With increased lamp usage, the lumen control system gradually increases power to the lamp to compensate for lamp lumen depreciation due to light-reducing mechanisms. By compensating for lamp lumen depreciation the lamp is operated at a constant mean or preset lumen output throughout the life of the lamp.

BACKGROUND

1. Field of the Invention

The present invention relates to lumen output control of a light source.More particularly, the invention provides a method and system forincreasing and decreasing a ballast output power, which is connected alight source, to provide a constant light output during the life of thelight source.

2. Description of the Related Art

Over time, the lumen output of a lamp continually decreases Lumen outputcan be defined as a unit of luminous flux equal to the light emitted ina unit solid angle by a uniform point source of one candle intensity. Asrelated to power, a lumen is 1/683 watts of radiant power at a frequencyof 540×10¹² Hertz. The lumen output degradation in the lamp can occurfor a variety of reasons, for example, lamp lumen depreciation, thelamp's interaction with a ballast, supply voltage variations, dirt ordust on the lamp, and the ambient temperature in a fixture. FIG. 1illustrates a lumen degradation curve for a typical quartz metal halidehigh intensity discharge (HID) lamp that uses a conventional ballast.FIG. 1 is a chart 100 illustrating two curves in relation to an X-axis102 (lamp operating hours) and a Y-axis 104 (lumens per lamp watt). Thecurve 106 illustrates the degradation curve for a magnetic constantwattage autotransformer (CWA) lamp and the curve 108 illustrates adegradation curve for a Prismatron™ lamp. As lamp operating hoursincrease for the lamp, the lumen output of the lamp decreases.

The decrease in lumen output occurs due to a variety of processes thatoccur within the lamp. One factor contributing to this decrease is aloss of chemicals that contributing to light output. These chemicals canbe lost through portions of the lamp structure, for example, an arccontainer Another factor contributing to light degradation is metalbeing deposited on an arc tube wall of the lamp. An HID lamp is startedby applying a very high voltage across an arc tube to break down highpressure gasses within the lamp into a conduction state. Following thisbreakdown, high current normally flows across a relatively low-voltagearc that heats the electrodes, which subsequently enter into thermionicemission. This tends to eject molecules of the metal electrode materialthat eventually condense on the wall of the arc tube, causing“blackening” and lowering the light transmission of the arc tube.

Due to such degradation in lumen output, many lighting applications aredesigned using a mean light level. The mean light level, or lamp'slumen, is defined when a HID lamp is at forty percent of its rated life.Typically to achieve a minimum light level emission, a lighting systemdesigner will design a lighting system at the mean light level. Once thelamp is at a point past the mean light level, replacement of the lamp isusually necessary to maintain a desired light output level.

In HID applications, a ballast is used to control the operating powerdelivered to a lamp. FIG. 2 is a block diagram 200 illustrating atypical ballast 202. The ballast 202 regulates the power to the lamp 204which is received as an input voltage from a power source (not shown).The ballast 202 also provides proper starting conditions for the lamp204 at start-up

Some ballast designs use magnetic transformers. As a result, the outputlevel of a lamp cannot be varied and is limited to an output of fullpower or some fixed output level lower than full power. Other ballastdesigns, such as electronic ballasts, provide for continuous variationof lamp voltage between full power and a predetermined lower limit.

However, a problem with conventional ballast systems, using the meanlight level to set a desired lamp output, is that the ballast initiallyconsumes additional power for the time period prior to achieving themean light level. Powering the lamp at full output prior to achievingthe mean light level causes an output higher than is necessary whichconsumes more power than necessary to provide the desired light output.

Accordingly, there is a need and desire for a ballast having a powerregulation technique for outputting power to a lamp, which will create aconstant lumen output from the lamp, thereby decreasing the powerconsumption of the lamp system.

SUMMARY

The present invention provides a constant output lumen control systemthat has the ability to provide a continuous lumen output from a lampover the lifetime of the lamp. The lighting system initially reduces thepower to the lamp, and subsequently varies the power delivered to thelamp to compensate for light-reducing mechanisms that will affect thelumen output of the lamp over time. By properly adjusting the powerdelivered to the lamp, the lighting system provides a constant lightoutput from the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments of the invention given below with reference to theaccompanying drawings.

FIG. 1 is a chart illustrating a lumen degradation curve for a typicalstandard metal halide HID lamp,

FIG. 2 is a block diagram illustrating a typical ballast design,

FIG. 3 is a block diagram illustrating a ballast design including lumencontrol circuitry in accordance with an embodiment of the invention,

FIG. 4 is a chart illustrating lamp output degradation as a function ofthe number of lamps starts,

FIG. 5 is a chart illustrating a re-lamp cycle for an HID lamp for lampreplacement detection,

FIG. 6 is a flow chart illustrating the process steps of an embodimentof the control circuitry of the invention,

FIG. 7 is a block diagram of an illumination system for implementing afirst exemplary embodiment of the present invention,

FIG. 8 is a chart illustrating power consumption of a conventionalballast and a ballast according to an embodiment of the invention,

FIG. 9A is a chart illustrating a re-ignition peak voltage as the lampvoltage vanes with time, and

FIG. 9B is a chart illustrating the relationship between a voltage crestfactor and lamp life.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and which is shown byway of illustration of specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical, and programming changes may be made withoutdeparting from scope of the present invention.

FIG. 3 is an exemplary illumination control system 300 employed in aballast 302. The ballast 302 includes a power factor correction circuit304, a power supply 306, a ballast control circuit 308, a lamp driver310, sense circuits 312, and an illumination control system 315. Theillumination control system 315 includes a computational control circuit314 and a non-volatile storage device 316. Non-volatile storage device316 may use any comparable non-volatile memory format, for example,dynamic random access memory (DRAM), flash memory, magneto-resistiverandom access memory (MRAM), etc. Computational control circuit 314 mayutilize a microprocessor or any other comparable processing device toconduct mathematical processing for adjusting power supplied to the lamp330 to achieve a constant lumen output from the lamp 330. Non-volatilestorage device 316 provides storage for various computational equations,mathematical constants, ballast operational software 318, timers 317,counters 319 and information regarding various lamp types, and theirspecific operational requirements which are used by the computationalcontrol circuit 314 during processing. The lamp 330 can be any type ofhigh intensity discharge lamp (HID), such as HID lamps that use highpressure mercury, high pressure sodium, or some other suitable gas.

The ballast control circuit 308 adjusts the power received from thepower supply 306 for use by the lamp 330. The ballast control circuit308 receives a lamp power setting signal and a lamp operational controlsignal from the computational control circuit 314. The ballast controlcircuit 308 also receives a lamp feedback signal from the sense circuits312 and provides operating power to the lamp driver 310. The lamp driver310 starts the lamp 330, receives operating power from ballast controlcircuit 308, and provides operating power to the sense circuits 312. Thelamp driver 310 receives a lamp on/off control signal from thecomputational control circuit 314 for use in discontinuing power beingsupplied to the lamp 330. The sense circuits 312 monitor the supplypower input to the lamp 330 and provide feedback about the operation ofthe lamp 330 to the computational control circuit 314 and the ballastcontrol circuit 308. The sense circuits 312 send a lamp current feedbacksignal and a lamp voltage feedback signal to the computational controlcircuit 314. The sense circuits 312 also send a lamp feedback signal tothe ballast control circuit 308 to monitor other important lampoperational parameters.

The illumination control system 315 utilizes various factors andparameters to determine a rate of degradation for a particular type oflamp 330. The parameters and factors are used to control the output ofthe lamp 330 over its lifecycle. For example, illumination controlsystem 315 may utilize operating hours (total hours the lamp has beenoperating) and lamp starts (total number of starting sequences for thelamp) to determine a rate of degradation of the lumen output of the lamp330. Other parameters may be considered in determining the degradationrate. For example, a stabilized lamp operating voltage, lamp re-ignitionvoltage, voltage crest factors, current crest factors, or combinationthereof may be used. Based upon the rate of degradation of the lamp 330,the illumination control system 315 adjusts the power supplied to thelamp 330 to provide a constant lumen output from the lamp 330.

The ballast operational software 318 resides in non-volatile storage 316and provides a variety of timers 317. For example, the timers 317include an accumulated lamp timer for measuring the number of operatinghours for the lamp 330, and a lamp warm-up timer for determining whenthe lamp 330 has achieved a stable state after starting for use by thecomputational control circuit 314. The ballast operational software 318also provides counters 319 for measuring the number of lamp starts forthe lamp 330. The ballast operational software 318 also controls theoperation of the ballast 302 and the power output by the ballast 302.

FIG. 4 illustrates a diagram 400, which compares the number of lampstarts to a percentage of lamp output power for the lamp 330. The X-axis402 represents a number of lamp starts for the lamp 330 and the Y-axis404 represents a percentage of output of the lamp 330. The output of thelamp 330, which is illustrated using curve 406, degrades due to lamplumen depreciation as the number of starts for the lamp 330 increases.

In calculating degradation due to the number of hours that the lamp 330is in operation, the computational control circuit 314 uses what isreferred to as a burnloss equation to determine lamp degradation due tooperating hours for use in calculating a dim level setting for the lamp330. The following second order polynomial equation determines the valuefor burnlossBurnloss=A×Hours² +B×Hours+C  Eq 1

The burnloss equation is stored in the non-volatile storage device 316along with constants A, B and C which are associated with the particulartype of lamp 330 being powered by the ballast 302. The constants A, B &C are derived from a least squares curve fitting using experimentaldata, based on light loss due to the number of operating hours of thelamp 330. The process of deriving the constants A, B and C could also bedone using a look-up table relating the variables, but such an approachwould require additional storage space in non-volatile storage device316.

In calculating degradation due to the number of lamp starts, thecomputational control circuit 314 uses what is referred to as astartloss equation to determine lamp degradation due to the number oflamp starts for use in calculating a dim level setting for theparticular type of lamp 330. The following second order polynomialequation determines the value for startloss.Startloss=D×Hours² +E×Hours+F  Eq 2

The startloss equation is stored in non-volatile storage device 316along with constants D, E and F which are associated with a particulartype of lamp 330 being powered by the ballast 302. The constants D, Eand F are derived and stored in non-volatile storage device 316 in asimilar manner as constants A, B and C.

The burnloss and startloss values for the lamp 330 are combined tocalculate an overall expected level of light loss at a given point inthe lifecycle of the lamp 330. A ratio is then calculated using theexpected level of light loss at a given point in the lifecycle of thelamp 330 and a predetermined lumen output target is stored innon-volatile storage 316. For example, an expected lamp output for agiven point (2000 hours) may be 95% of the initial lamp output, whilethe predetermined lumen output target is 85%. Thus, the output wattageto the lamp 330 is decreased by an appropriate amount to reduce thelight output of the lamp 330 to the predetermined lumen output target.Although the target lumen output of the lamp 330 may be set to anyreasonable lumen output, two meaningful output settings which may beused are an end of life lumen output and a mean lumen output. The meanlumen output is typically the average light output after 40% of theexpected life of the lamp 330 has elapsed and is usually set by themanufacturer of the lamp 330.

By using the ratio of expected lumen output to current lumen output, thepower supplied to the lamp 330 may be adjusted by the illuminationcontrol system 315 to set an appropriate source wattage for the lamp330. For example, if the lamp 330 is a quartz metal halide HID lamp, alumen output for the illumination control system 315 would be varied 1 8times a change in wattage due to the relationship between the lampwattage and the delivered light output for the particular type of lamp330. Therefore, the wattage from the ballast 302 to the lamp 330 ischanged by a ratio of 1/18 to obtain a desired constant lumen output.Thus, as the number of operating hours and lamp starts accumulate, theillumination control system 315 continually evaluates the degradation ofthe lamp 330 to compensate for lamp lumen degradation by increasing thewattage output supplied from the ballast 302 to the lamp 330. When thelamp 330 degrades to a point at which the lamp 330 requires more powerthan its maximum power rating (100%) to maintain the desired lumenoutput level, the illumination control circuit 315 will limit the poweroutput by the ballast 302 to the maximum power rating of the lamp 330.By limiting the lamp 330 to its maximum power rating, safety is improvedbecause the lamp 330 is not overdriven which could damage the circuitrywithin the ballast 302 and the lamp 330. Once the lifecycle of the lamp330 is completed, the lamp 330 is subsequently replaced.

After the lamp 330 is replaced, values such as the number of operatinghours and the number of lamp starts stored in the non-volatile storagedevice 316 are reset. Although it is possible to reset the non-volatilestorage device 316 manually, a reset means using a form of lampreplacement detection may be employed. The lamp replacement detectiontechnique may be employed using software included in ballast operationalsoftware 318 which is stored in the non-volatile storage device 316 foruse by the computational control circuit 314. By comparing the measuredlamp voltage of the lamp 330 to the lamp voltage stored in memory, thecomputational control circuit 314 determines if a change in lamp voltagehas occurred which would indicate that the lamp 330 has been replaced.

Thus, a lamp replacement detection technique may utilize the fact thatas a lamp ages, many electrical variables associated with the lampchange. For example, a root mean squared (RMS) voltage across the lamp330 and a re-ignition voltage for the lamp 330 change over time. Thelamp replacement detection technique uses the software included inballast operational software 318 to store these voltages and othervariables in the non-volatile storage device 316. Each time the lamp 330is started, a stabilized lamp voltage is compared to a stored stabilizedlamp voltage setting. If a step in voltage is greater than apredetermined threshold level stored in the non-volatile storage device316, then it is determined that the lamp 330 has been replaced. Forexample, if a decrease of 5 volts in lamp voltage is determined by thecomputational control circuit 314 after the lamp voltage has stabilized,the lamp 330 is determined to have been replaced. After such adetermination, the number of operating hours and the number of lampsstarts are reset in the non-volatile storage device 316.

FIG. 5 illustrates the above described replacement technique using thecomparison of lamp start voltages. The chart 500 graphs a percent relampcycle 502 versus a lamp start voltage 504 using curve 506. During eachstart, the voltage of the lamp 330 is obtained and compared to a lampvoltage stored in the non-volatile storage device 316 from the previouslamp start. If the lamp voltage step between starts is greater than thepredetermined threshold, for example, a step from 160 volts (508) to 100volts (510), the illumination control system 315 determines that thelamp 330 has been replaced since the stabilized lamp voltage is reducedby 60 volts from a previous lamp operation. Subsequently, the number ofoperating hours and the number of lamp starts stored in the non-volatilestorage device 316 are reset. Those skilled in the art will recognizethere are many other comparable means to perform the lamp replacementdetection described above.

FIG. 6 is flow diagram 600 of process steps implemented by theillumination control system 315. The blocks in the flow diagram 600 maybe performed in the order shown, out of the order shown, or may beperformed in parallel. At step 602, power is applied to the ballast 302turning on the lamp 330. Next, at step 604, the lamp 330 is adjusted tofull power At step 606, ballast 302 obtains a variety of constant lumenoutput control (CLO) values, for example, total lamp starts, historiclamp voltage and lamp life constants based on the particular type oflamp 330 used from the non-volatile storage device 316. At step 608, theballast 302 starts a lamp warm-up timer having a predetermined warm-uptime setting, for example, 20 minutes. At step 610, the accumulated lamptimer is started. The lamp warm-up timer and accumulated lamp timer arecreated using the timers 317 which are stored in the non-volatilestorage device 316 for use by the computational control circuit 314.Next, at step 612, the ballast 302 increments the counter 319 (FIG. 3)measuring the number of lamp starts and stores the new lamp start valuein the non-volatile storage device 316. At step 614, the ballast 302determines whether the predetermined warm-up time period has elapsed toassure the lamp wattage and voltage has stabilized. If the warm-up timeperiod has not elapsed, the process returns to step 614 At step 616, ifthe warm-up time period has elapsed, the ballast 302 determines whetherthe lamp 330 has been replaced using the technique described in FIG. 5.

If the lamp 330 has been replaced, then, at step 618, the ballast 302resets the number of operating hours and the number of lamp starts totheir predetermined reset values. For example, operating hours areassigned a value of 10 and the number of starts is assigned a valueof 1. If the lamp 330 has not been replaced, the process proceeds tostep 620 where the ballast 302 writes the current value for the numberof operating hours, the number of lamp starts and a lamp start voltagebeing used by the lamp 330 into the non-volatile storage device 316.

At step 622, the ballast 302 determines the projected lamp lumen outputfor the lamp 330 based on the degradation curve stored in thenon-volatile storage device 316 for the particular lamp type.Subsequently, at step 624, the degradation of the lamp due to the numberof starts is derived from the stored compensation curve for theparticular type of lamp 330 being utilized At step 626, the targetoutput lumens of the lamp 330 is ratioed to the calculated currentlumens to adjust the power supplied to the lamp 330 to maintain aconstant lumen output from the lamp 330 At step 628, the ballast 302determines the actual power setting, in watts, to which the lamp 330should be adjusted to provide the target lumens by converting outputlumens to watts. The conversion is calculated from a light output versuspower curve for the lamp type 330 being utilized. At step 630, theballast 302 adjusts the output wattage to the lamp 330 by setting aninternal reduced power level setting.

Thus, by using the ballast 302 which can adjust power input to the lamp330, an illumination system may be implemented which is efficient andcost-effective.

As mentioned above, the ballast 302 may also utilize the stabilized lampoperating voltage to maintain a constant lumen output for the lamp 330.Instead of combining the results of the burnloss and startlossequations, the computational control circuit 314 calculates a value forwhat is referred to as Slov, and combines the Slov and startlossequations to maintain a constant lumen output for the lamp 330. Slovrepresents the stabilized lamp operating voltage and could be determinedby using the following second order polynomial equationSlov=G×Hours² +H×Hours+I

The value for Slov is stored in non-volatile storage device 316 alongwith constants G, H and I which are associated with a particular type oflamp 330 being powered by the ballast 302. The constants G, H and I arederived and stored in non-volatile storage device 316 in a similarmanner as constants A, B and C.

FIG. 7 illustrates an illumination system 700 using multiple ballasts302. Illumination system 700 includes multiple ballasts 302 eachconnected to power supply 702 for controlling the lumen output of a lamp330 connected to each ballast 302. Thus, illumination system 700utilizes multiple ballasts 302 and lamps 330 to illuminate larger areaswhich could be used in a variety of lighting applications.

FIG. 8 is a diagram 800 illustrating power consumption of a lamp 330using a conventional ballast and the ballast 302. In FIG. 8, a timecomponent (X-axis 802) and a percent lamp power component (Y-axis 804)are used to compare a constant light output 806 produced by the lamp 330using supply power from the ballast 302 versus light output 808 from thelamp 330 using supply power from a conventional ballast. Because aconventional ballast cannot adjust power input to the lamp 330, theconventional ballast provides full power to the lamp 330 when full poweris not needed. The area indicated at 810 between curves 806 and 808illustrates power wasted when a lamp 330 is conventionally controlled.Thus, power consumed by a lamp 330 that is controlled by a conventionalballast exceeds the power consumed by a lamp 330 that is controlled bythe ballast 302. By adjusting the power output from the ballast 302, thelamp 330 is provided with only enough power to maintain an establishedlumen output level. Thus, power costs are reduced since the ballast 302does not overdrive the lamp 330 by supplying more power than isrequired.

As mentioned with reference to FIG. 3, another alternative to burninghours and lamp starts utilizes the re-ignition voltage, or morespecifically the voltage crest factor (VCF). The re-ignition of the lampdischarge occurs each time the lamp current changes polarity. As aresult, the arc and electron flow must be re-established, which takes afinite amount of time. This time creates a resultant arc impedancechange, which results in an instantaneous rise in lamp voltage that islimited by the instantaneous open circuit voltage of the ballast. Thetime and voltage necessary to re-establish the arc is dependent on theability of the electrode to supply electrons and continue therecombination process. As the HID lamp 330 ages, the ability of theelectrode and fill gas to provide and transport electrons decreases. Theresultant magnitude of the voltage peak, measured at zero currentcrossing, is called the re-ignition voltage, which subsequentlyincreases. Turning now to FIG. 9A, the peak re-ignition voltage for anew HID lamp is shown at reference numeral 910. After some time, thepeak re-ignition voltage for this aged HID lamp is shown at referencenumeral 920. Hence the peak re-ignition voltage is a factor that vaneswith lamp age.

The VCF is defined using the peak re-ignition and rms lamp operatingvoltage that can be used for monitoring lamp life. More specifically,the VCF is the ratio of the peak re-ignition voltage to the rms voltageof the lamp operating voltage. Because the VCF changes as the peakre-ignition voltage changes with lamp age, the VCF vanes with lamp age.The graph 930 in FIG. 9B illustrates the variation of the VCF with lamplife. Thus, monitoring of the VCF can be used as a parameter to estimatethe burning hours of the lamp 330 and provide data to the computationalcontrol 314 to adjust the power to the lamp 330 for maintaining constantlumen output.

While the invention has been described in detail in connection with anexemplary embodiment, it should be understood that the invention is notlimited to the above-disclosed embodiment. Rather, the invention can bemodified to incorporate any number of variations, alternations,substitutions, or equivalent arrangements not heretofore described, butwhich are commensurate with the spent and scope of the invention. Inparticular, the specific embodiments of the constant lumen outputcontrol system described should be taken as exemplary and not limiting.For example, the ballast 302 may also determine lumen degradation oflamp 330 by measuring the change in the RMS voltage, voltage and currentcrest factors, re-ignition voltage or combination of these parameters oflamp 330 or by monitoring the lumens emanating from the lamp 330, bylumens received at a task being illuminated by the lamp 330.Accordingly, the invention is not limited by the foregoing descriptionor drawings, but is only limited by the scope of the appended claims.

1. A method of providing constant lumen output control to a lumen outputdevice, comprising: determining a number of operating hours for a lamp,determining a number of lamp starts for the lamp, creating a degradationvalue by combining the number of operating hours and the number of lampstarts, forming an output ratio for outputting power to the lamp usingthe degradation value and a target lamp output, and setting a reducedpower level for the lamp using the output ratio.
 2. The method of claim1, wherein the target lamp output is specific to the particular type oflamp.
 3. The method of claim 1, wherein the reduced power level isadjusted throughout the life of the lamp to maintain a constant lumenoutput.
 4. The method of claim 3, wherein the adjustment compensates forlamp degradation within the lamp.
 5. The method of claim 1, furthercomprising resetting the number of operating hours and the number oflamp starts when the lamp is replaced.
 6. The method of claim 5, whereina lamp voltage comparison is used to determine when the lamp has beenreplaced.
 7. The method of claim 1, wherein the step of determining thenumber of operating hours, the step of determining the number of lampstarts, the step of creating a degradation value, the step of forming anoutput ratio and the step of setting a reduced power level are performedby a processor.
 8. A lumen output control circuit, comprising: a timerfor measuring a number of operating hours for a lamp, a counter formeasuring a number of lamp starts for the lamp, a non-volatile storagedevice having computer-executable instructions, wherein the non-volatilestorage device is configured to store the number of operating hours andthe number of lamp starts; a processor functionally coupled to thenon-volatile storage device and configured, by the computer-executableinstructions, to create a degradation value by combining the number ofoperating hours and the number of lamp starts, and to form an outputratio for outputting power to the lamp using the degradation value and atarget lamp output.
 9. The circuit of claim 8, wherein the processor isconfigured to set a reduced power level for the lamp using the outputratio.
 10. The circuit of claim 9, wherein the reduced power level isadjusted throughout the life of the lamp to maintain a constant lumenoutput.
 11. The circuit of claim 8, wherein the target lamp output isspecific to the particular type of lamp.
 12. The circuit of claim 8,wherein the non-volatile storage device stores a lamp voltage for eachlamp start for the lamp, and wherein a lamp voltage comparison is usedto determine when the lamp has been replaced.